We assume that phone calls arrive according to a Poisson process with rate 2. If I know that up to time t = 1 there is one call only, how likely is it that it arrived after t = 0.75? If I know that up to time t = 1 two calls arrived, how likely is it that the second arrived after t = 0.75?
So for the first problem I set that
$P(S_1>0.75/X(1)=1)$, with $S_1$, the time it takes for the first event to occur, and $X(1)$, the number of events that occurred up to time 1. But then I don't quite see how to handle it.